It is proposed to continue studies on the biochemical characterization of Factor V and to investigate its role in the blood coagulation process. In particular, it is planned to define those conditions which lead to the stability of Factor V and Factor Va preparations. The presence of trace amounts of Factor VIII are now considered to contribute to the instability of Factor V, and more especially Factor Va preparations. The possibility that Factor Va is a substrate for Factor VIIIa will be explored in some depth. Continuation of an on-going study on Factor V conversion to Factor Va will center on identification of the components released when Factor V is treated with highly purified RVV-V or thrombin. In addition, the interaction of Factor V and Factor Va with phospholipids will be followed. It is hoped to define through the use of synthetic (and labeled) phospholipids whether there is a site for binding on the Va or V molecule which is important in the interaction with Factor Xa and Ca2 ion (to form prothrombinase) and also if there is a binding site(s) for phospholipid important in the activity of the Factor V(Va) in the coagulation process.